The Ruling on Making up for Witr

Question :

If I slept through the Witr prayer and I did not perform it during the night, can I make it up, and when?

Answer:

The Sunnah is to make it up after sunrise and before it reaches its zenith, praying an even number, not Witr (an odd number). So if your custom is to offer three Rak'ahs as Witr at night and you slept through it or forgot it, it is lawful for you then to pray four Rak'ahs during the day with two Taslims. If you normally offer five Rak'ahs as Witr at night, but you slept through it or forgot it, it is lawful for you to pray six Rak'ahs during the day with three Taslims. The ruling is the same if one prays more than this, based upon the authentic Hadith narrated on the authority of 'A'ishah, may Allah be pleased with her, in which she said: "When Allah's Messenger was unable to pray the night prayer due to sleep or illness, he used to pray twelve Rak'ahs during the day."

And his Witr usually consisted of eleven Rak'ahs, and the Sunnah is to make up for it by praying an even number, in units of two Rak'ahs, based upon this noble Hadith and the words of the Prophet:
"The night prayer and the day prayer are in units of two (Rak'ahs).

The essence of this Hadith is to be found in the Two Sahihs on the authority of Ibn 'Umar, but without mention of the day, although this addition is authentic by those whom we mentioned earlier, i.e. Ahmad, and the compilers of the Sunan. And Allah is the Granter of success.

Source:
Ibn Baz
Fatawa Islamiyah, Vol. 2 Pages 315-316